Testing of digital devices by the signature analysis technique involves placing of output signals from the card under test into a shift register. The output signals are produced as a result of input signals which are applied to the card under test according to a predetermined sequence. The result of the test contained in the shift register represents the product of all of the output signals produced over a period of time although the resulting "signature" may be, for example, only 16 bits long if a 16 character shift register is used. If the circuit operates properly, those 16 bits should correspond to an expected 16 bits and the operation of the card can be termed a success. It is crucial in this test that the production of the signature begin at a specific known time in the operation of the card under test and end at a specific known time. If the start or stop signal is off by even one bit, the resulting signature will be incorrect and a properly operating card might be considered faulty. Also, it is crucial that the signature clock does not coincide with transitions in the signal from the card under test for this could cause different readings for a properly operating card (unstable signature readings).
U.S. Pat. Nos. 3,976,864 and 4,192,451 disclose apparatus for testing digital equipment with the signature analysis technique. In each of these previous arrangements, however, it is a requirement that the device under test provide the necessary start and stop signals and signature clock in order to operate the signature generator. The result is a limitation on the usefulness of the signature analysis technique since some integrated circuits such as, for example, a single chip microprocessor, may not be provided with the necessary testability requirements for signature analysis. That is to say, the clock cycles of certain circuits are not available on I/O pins.